Page 85 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Solved Problems 77

• A continuous trait can have any value of expression
essential concepts
between two extremes. Most traits of this type are
polygenic, that is, determined by the interactions of
• In incomplete penetrance, a phenotype is expressed in
fewer than 100% of individuals having the same multiple genes.
genotype. In variable expressivity, a phenotype is • The environment and random events can interact with
expressed at different levels among individuals with the genes to influence the expression of many so-called
same genotype. multifactorial traits.

WHAT’S NEXT

Part of Mendel’s genius was to look at the genetic basis of this chapter. Each embellishment extends the range of
variation through a very narrow window, focusing his first Mendelian analysis and deepens our understanding of the
glimpse of the mechanisms of inheritance on simple yet genetic basis of variation. And no matter how broad the
fundamental phenomena. Mendel worked on just a handful view, Mendel’s basic conclusions, embodied in his first law
of traits in inbred populations of one species. For each trait, of segregation, remain valid.
he manipulated one gene with one completely dominant But what about Mendel’s second law that genes assort
and one recessive allele that determined two distinguish- independently? As it turns out, its application is not as
able, or discontinuous, phenotypes. Both the dominant and universal as that of the law of segregation. Many genes do
recessive alleles showed complete penetrance and negligi- assort independently, but some do not; rather, the two
ble differences in expressivity. genes appear to be linked and transmitted together from
In the first few decades of the twentieth century, many generation to generation. An understanding of this fact
biologists questioned the general applicability of Mende- emerged from studies that located Mendel’s hereditary
lian analysis, for it seemed to shed little light on the com- units, the genes, in specific cellular organelles, the chro-
plex inheritance patterns of most plant and animal traits or mosomes. In describing how researchers deduced that
on the mechanisms producing continuous variation. Simple genes travel with chromosomes, Chapter 4 establishes the
embellishments, however, clarified the genetic basis of physical basis of inheritance, including the segregation of
continuous variation and provided explanations for other alleles, and clarifies why some genes assort independently
apparent exceptions to Mendelian analysis as described in while others do not.

SOLVED PROBLEMS

I. Imagine you purchased an albino mouse (genotype informative.) What alleles of the B gene should the test
cc) in a pet store. The c allele is epistatic to other coat mouse carry? To make this decision, work through the ex-
color genes. How would you go about determining pected results using each of the possible genotypes.
the genotype of this mouse at the brown locus? (In
pigmented mice, BB and Bb are black, bb is brown.) Test mouse Albino Expected non-
genotype mouse albino progeny
Answer BB × BB all black
× Bb all black
This problem requires an understanding of gene interactions, × bb all black
specifically epistasis. You have been placed in the role of Bb × BB all black
experimenter and need to design crosses that will answer the × Bb 3/4 black, 1/4 brown
question. To determine the alleles of the B gene present, you × bb 1/2 black, 1/2 brown
need to eliminate the blocking action of the cc genotype. bb × BB all black
1/2 black, 1/2 brown
×
Bb
Because only the recessive c allele is epistatic, when a C al- × bb all brown
lele is present, no epistasis will occur. To introduce a C allele
during the mating, the test mouse you mate to your albino From these hypothetical crosses, you can see that a test
can have the genotype CC or Cc. (If the mouse is Cc, half mouse with either the Bb or bb genotype would yield
the progeny will be albino and will not contribute useful in- distinct outcomes for each of the three possible albino
formation, but the non-albinos from this cross would be mouse genotypes. However, a bb test mouse would be more
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